1. Field of the Invention
This invention deals generally with industrial machines for comminuting and disintegrating solid material. In particular, this invention relates to grinding machines for reducing scrap tires into crumb rubber.
Hundreds of millions of scrap tires are generated every year in the United States. At one time the majority of these tires would end up in landfills. Current estimates are that as many as 300 million scrap tires may be stockpiled, but millions more may already be buried in landfills where they remain uncounted. Stockpiling scrap tires poses serious environmental and health risks stemming from the risk of large, uncontrolled fires in tire piles and spread of disease by insects and vermin for whom tire piles serve as prime breeding grounds. Recycling addresses these risks and allows recovery of a significant economic resource.
The waste stream of used tires continues to grow. In light of concern for this growing problem, tire recycling has become increasingly important. It encourages research for economical uses for recycled rubber and drives the search for improvements in recycling technology and equipment. In 2003, more than 290 million scrap tires were generated in the United States. Of those, nearly 100 million were recycled into new products and 130 million were reused as tire-derived fuel in various industrial facilities.
Uses for recycled tires fall into one of three major categories: tire-derived fuel (TDF), civil engineering applications, and ground rubber applications. Civil engineering and TDF applications account for roughly half of the scrap tire waste stream. Scrap tires for TDF and civil engineering applications are shredded into chips approximately three inches square. Fibrous cord and steel wire used in belting and bead reinforcement typically remain embedded in the rubber though the bead portion is often removed and scrapped. While these applications dramatically reduce the volume of scrap tires, both considered low value applications since they recovery relatively little of the energy investment in the tires.
The third major use for scrap tires is applications requiring ground rubber, that is rubber comminuted to sizes on the order of 10 mesh or smaller. Ground rubber applications represent the highest value-added uses of scrap tires and include applications in which the rubber particles are incorporated in a variety of products such as asphalt modifications, molded products, sport surfacing and manufacturing of new tires. The use of scrap tires in ground rubber applications makes far more economic and environmental sense than TDF and civil engineering applications.
It is desirable to have an efficient and durable tire rubber granulator apparatus capable of comminuting large volumes of scrap tires into crumb rubber efficiently. It is also desirable that the tire rubber granulator apparatus be capable of minimizing the amount of rubber wastage during the wire and fibrous material separation process. It is further desirable that the tire rubber granulator apparatus be adjustable to accept a variety of sizes of input material and to produce various sizes of crumb rubber. Finally, it is desirable that the tire rubber granulator apparatus be easily alterable to enable comminuting a range of recyclable materials beyond scrap tires.
2. Description of the Prior Art
Producing ground rubber, also known as crumb rubber, is a costly process. Tire rubber is highly resilient and therefore difficult to grind efficiently. Multiple processing steps are often required to obtain crumb rubber of a commercially viable size and quality. Bead wire, steel belts and fiber are embedded in the rubber and must be separated from the rubber crumb. The value of rubber increases tenfold when the wire and fiber is removed, but conventional equipment capable of efficiently grinding scrap tires and removing the embedded wire does not fully capitalize on this difference. Conventional tire grinding apparatus typically discard portions of scrap tires to avoid processing the wire and waste a significant portion of the scrap rubber.
Tires are typically recycled by first removing the tire bead where the heaviest gauge wire is contained using a dedicated machine. With the bead removed, the remainder of the tire carcass can be shredded or ground as desired. U.S. Pat. No. 6,249,949 by Cross, Jr. et al. and U.S. Pat. No. 5,768,764 by Batt disclose apparatus specifically designed to remove tire beads from scrap tires. While removing and scrapping tire beads prior to tire comminuting eliminates the larger gauge wire from the recycling input, tire beads also contain significant amount of rubber. Scrapping the bead results in a high level of rubber wastage and increased scrap disposal costs.
Once the bead is removed, there are several methods for further comminuting the scrap tires. These are generally categorized as shredding or slicing methods, and grinding methods. Conventional rubber recycling grinders typically contain a pair of counter-rotating, parallel rollers with a small separation between them and/or interacting teeth axially spaced along the rollers. Material to be ground is forced between the rollers and sheared by interaction of the teeth. The cutting elements of one roller extend into spaces between the cutting elements on the opposing roller so that the cutting elements interact to cut rubber as it is fed into the spaces. Such machines are designed to reduce whole tires to chips on the order of three inches square. U.S. Pat. No. 4,614,308 by Barclay, U.S. Pat. No. 5,048,764 by Flament, and U.S. Pat. No. 6,036,128 by Cramer disclose various machines typical in the shredding art. Other similar apparatus rely on knife-like teeth to slice the tires into tire chips. U.S. Pat. No. 5,024,386 by Morris discloses an apparatus utilizing slicing to reduce the scrap tires. Two limitations of shredding/slicing technology are the inability of such machines to further reduce tire chips to crumb rubber and that wire beads must typically removed prior to shredding since the heavy wire common in tire beads will dull or damage the cutting edges thereby increasing maintenance requirements on the machine. Additionally, these machines require large amounts of energy to overcome the resiliency of the rubber and break the rubber into smaller pieces making them expensive to operate.
Grinding apparatus are used when the desired output is granulated rubber, also known as rubber crumb. Some grinders employ two-roll, grooved mills in which the rolls turn at slightly different rates. The speed differential creates a shear on the material which combined with the surface texture of the rolls rips the rubber into smaller pieces. Multiple passes through progressively tighter clearances in conjunction with screens to retain the larger pieces within the grinding apparatus result in crumb rubber of a desired size. Because of the tight clearances between the rollers, many conventional tire grinding machines require that tires first be cut into smaller pieces, such as TDF, prior to the grinding process and most require that scrap tires be debeaded prior to processing. Roller grinders typically require large power inputs to overcome the resiliency of the rubber. Advancements in grinding technology have targeted reducing power input requirements, developed machines capable of separating wire and fiber from the rubber crumb, and increasing the throughput of the machine. U.S. Pat. No. 5,624,078 by Bredbeck and U.S. Pat. No. 5,921,480 by Wenzel are typical in this art.